Study on the effect of ferrous ion release from the disposal tank on the expansion performance of bentonite buffer barrier

Bentonite, as a buffer/backfill material, plays a vital role in ensuring the long-term safety and stability of high-level radioactive waste (HLW) repositories due to its swelling and self-sealing properties. However, Fe(II), a corrosion product released from disposal canisters, may alter the mineralogical composition of bentonite and consequently degrade its swelling performance. Despite its importance, limited studies have addressed this issue. This study aims to elucidate the impact of Fe(II) release on the swelling behavior of bentonite and to predict the spatiotemporal evolution of buffer swelling performance during repository operation. A custom-designed swelling apparatus and a corrosion-resistant flexible-wall permeameter were employed to conduct swelling and diffusion tests on compacted GMZ24 bentonite under varying Fe(II) concentrations. Furthermore, an artificial neural network algorithm was applied to predict the spatiotemporal evolution of swelling pressure for GMZ24 bentonite and GMZ001 bentonite (the candidate buffer material in China) during the repository service period. The results indicate that:(1) The swelling pressure of GMZ24 bentonite exhibits an exponential decay trend with increasing Fe(II) concentration. Under realistic conditions (Fe(II) concentration of 0.089−0.24 mol/L), the maximum swelling pressure decreased by 13.9%−20%; under extreme conditions with FeCl₂ at saturated solubility, the reduction reached 33.3%. (2) As Fe(II) concentration increased from 0 to 2.4 mol/L, the permeability coefficient of GMZ24 bentonite decreased from 3.35×10⁻¹⁰ m/s to 2.6×10⁻¹¹ m/s. The apparent diffusion coefficient of Fe(II) in GMZ24 bentonite was determined to be 3.4×10⁻¹² m²/s, and the effective diffusion coefficient was 4.1×10⁻⁶ m²/s. (3) The machine learning model indicated that, for GMZ24 bentonite, the swelling pressure in the vicinity of the canister-buffer interface would fall below 1 MPa after 90 years, and in the buffer-host rock interface region after 1000 years. In contrast, GMZ001 bentonite, with a higher montmorillonite content, maintained adequate swelling pressure throughout the entire repository service period. The results provide scientific insights for the design of buffer systems and the optimization of in-situ testing schemes in China’s HLW geological disposal repositories.